GB2353638A

GB2353638A - Low frequency electromagnetic absorption surface

Info

Publication number: GB2353638A
Application number: GB9920009A
Authority: GB
Inventors: Christopher Robert Lawrence; John Roy Sambles; Alistair Paul Hibbins
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1999-08-25
Filing date: 1999-08-25
Publication date: 2001-02-28
Also published as: CA2380744C; AU6461800A; EP1206814B1; GB2370420A; GB9920009D0; GB0201077D0; EP1206814A1; CA2380744A1; JP2003508945A; GB2370420B; WO2001015274A1; ATE258338T1; DE60007877D1; DE60007877T2; US6642881B1

Abstract

A microwave radiation absorber comprises a metallic substrate 1 with a dielectric layer 2 coated thereon. The dielectric layer has a textured/patterned surface which is preferably wave like. The preferred dielectric material is petroleum wax and the waveform may then be made by combing the wax after application to the metal surface. The dielectric layer may comprise tape strips, may include doping agents and may be applied to buildings, vehicles or solar panels.

Description

2353638 Low frequency electromagnetic absorption surface The invention

relates to low-frequency electromagnetic absorption surfaces.

Surface plasmon polaritons (SPPs) are charge density oscillations induced at the surface of a metal at a metal-dielectric interface when photons are coupled to the mode in the correct manner. The momentum of the incident photons must be boosted if the resonant condition is to be met, and this can be achieved by corrugating the metal to form a diffraction grating. The energy is absorbed by the metal due to damping of the charge density oscillation (i.e. charge collisions lead to heating in the metal), and hence the plasmons cannot convert back to photons for re-emission. In this manner the reflectivity of the metal is reduced when photons are absorbed. This phenomenon is well known at visible frequencies, and forms the basis of many sensor designs.

At microwave frequencies any SPPs that are excited at the surface of the metal will propagate without loss because the charge density oscillations are virtually undamped (i.e. the photon energy cannot be absorbed). Instead of being absorbed, the SPPs will skim the surface until they are converted back to photons at a diffractive feature such as an edge, a curve or the original diffraction grating. Hence the radiation will eventually be re-emitted, and possibly back towards the radiation source. In order to reduce these stray emissions, lossy materials are used as surface coatings to absorb any SPPs that are excited, and methods to prevent the excitation of the modes are sought.

A flat metal plate is a highly efficient microwave reflector that will not normally support SPPS. If it is desired that the plate should absorb all of the energy that fails upon it then absorbing materials are used as surface coatings. Electrically-absorbing materials need to be placed a specific distances from the metal, the shortest of which is a quarter of the wavelength to be absorbed. In the case of magnetic absorbers these are placed directly onto the metal plate, but they are far heavier than electric absorbers. Hence weight and bulk considerations need to be taken into account.

Prior art grating coupling geometry uses a corrugated metal/dielectric interface and when grating coupled in this way, the SPP propagates along this corrugated boundary. Since the

2 periodic surface may scatter energy associated with the mode into diffracted orders, the propagation length of the mode is reduced. The disadvantage is that complicated profiles cannot easily be made on a metal layer and expensive and complicated techniques of machining metal are required. In addition, the SPP that propagates along the textured surface may only be radiatively damped since the media either side of the boundary are usually nonabsorbing.

It is the object of the invention to provide for a relatively thin, lightweight, broadband absorber, which is relatively simple to fabricate and incorporates a second damping mechanism by which the SPP may decay.

The invention comprises radiation absorber comprising a substrate having free charges and a dielectric layer coated onto said surface wherein the dielectric layer has a textured patterned surface.

Preferably the first substrate is metallic.

Such dielectric gatings (wax) placed onto the metal plate will excite SPPs. The grating can potentially be far thinner than a quarter of a wavelength, and could even be applied in the form of sticky tapes at set spacing. Complicated profiles can easily be carved into soft dielectric (e.g. wax) layers.

In radiation absorbers according to the invention, there are two independent damping process that acts on the SPP as it propagates along the boundary. Firstly, the mechanism that allows radiation to couple into the SPP (i.e. the grating) will also allow the mode to radiatively decay. Secondly, although the top and bottom semi-infinite media (air and metal respectively) are effectively non-absorbing, at these frequencies, this may not be true for dielectrics, such as wax. Since the evanescent fields associated with the SPP mode penetrate the wax, any loss mechanisms within this overlayer will contribute a term to the damping of the mode. Both of these damping terms will contribute to the width of the surface plasmon resonance and will also have a similar effect on any guided modes propagating in the system.

Preferably the dielectric layer is doped with an appropriate absorbing material (e.g. ferrite particles, carbon fibre). In this instance, the SPPs are absorbed by the grating rather than the metal and absorption occurs across a range of wavelengths.

3 The invention will now be described with reference to the following figures of which Figure 1 shows an embodiment of the invention comprising a metal substrate having a dielectric layer of petroleum wax with a profiled surface.

Figure 2 shows an arrangement used to record reflectivity from the sample.

Fig. 3 illustrates a polar grey-scale map of the normalised Rpp, Rps, and %, signals from the sample as a function of frequency and azimuthal angle of incidence.

Figure 1 shows the substrate 1 having a dielectric layer of petroleum wax 2 with a profiled surface. This profile is corrugated (sinusoidal) and having pitch p, amplitude a, and dielectric thickness t. The sinusoidal top interface profile A(x)=acos 2M/Xg, where t z:: 2.6mm, a R 1.5 nim and Xg = 15 mm.

The sample is prepared by filling a metallic, square tray of side approximately 400 mm and depth 5 min with hot wax and allowing it to cool. A metallic "comb" of the desired sinusoidal interface profile is manufactured using a computer-aided design and manufacture technique. It is used to remove unwanted wax from the sample by carefully dragging it across the surface until the required grating profile is obtained.

Figure 2 shows an arrangement used to record reflectivity from the sample. A transmitting horn 3 is placed at the focus of a 2m focal length mirror 4 to collimate the beam therefrom. A second mirror 5 is positioned to collect the specularly reflected beam from the grating and focus it at the detector 6. The dielectric grating on the metallic substrate is show designated together by reference numeral 7. Variation of the magnitude of the incident wave - vector in the plane of the grating may be achieved by scanning either wavelength (X) or the angle of incidence (0, g). The reflectivity data is recorded as a function of wavelength between 7.5 and 1 lmm, and over the azimuthal angle ((p) range from 0' to 90' at a fixed polar angle of incidence, 0 47'. The source and receiving horn antennae are set to pass either p(transverse magnetic, TM), or s- (transverse electric, TE) polarizations, defined with respect to the plane of incidence. This enables the measurement of Rpp, Rp, %, and Rp reflectivities.

4 The resulting wavelength- and angle-dependent reflectivities from the sample are normalised by comparison with the reflected signal from a flat metal plate.

Fig. 3 illustrates a polar grey-scale map of the normalised Rpp, Rp,, and %, signals from the sample as a function of frequency and azimuthal angle of incidence. Since the profile of the grating is non-blazed, the results from the two polarisation conversion scans are identical, and hence we do not illustrate the Rp response.

Fig. 4 shows a series of experimental data sets of reflectivity against azimuthal angle (E) at wavelengths of (a) 7.5mm, (b) 8.5 min, (c) 9.5 min and (d) 10.5 mm, showing the Rpp, R,,, Rps and R,, signals respectively.

Figures 5 and 6 illustrate the effect of the imaginary part of the permittivity of the dielectric layer on the modelled R,, response and degree of absorption of the sample at 1 Imin wavelength.

Variables of frequency, dielectric thickness and profile shape can be selected to control the coupling strength (of the incident radiation to the surface plasmon). The corrugated air- dielectric boundary excites diffracted orders which provide the required enhanced momentum to couple radiation to the SPI? associated with the wax interface The diffracted SPP (TM) modes propagate along the metal-wax interface. Note that the coupling strength to the SPP decreases to zero as (p = 0' is approached. This is because the incident TE field has no component of electric field acting perpendicular to the grating surface and hence cannot create the necessary surface charge. In other words, the excitation of the modes is polarisation dependent in the case of the single-period textured surface.

The evanescent fields associated with the SPP will sample the wax layer and will penetrate into the air half-space. Therefore, the dispersion of the SPP will be dependent on an effective refractive index (n''Wx) since the degree of penetration into the air is governed by the thickness of the wax overlayer. In addition the excitation of guided modes within the dielectric layer also becomes possible where, in contrast to the SPP, the dispersion of these modes is governed by the true refractive index of the layer, nwa,,, where n,,.,, < krm < n,Xk,,.. In a similar manner to the SPP, the guided mode also moves away from the pseudo-critical edge as the wax thickness is increased.

Fig. 4 shows a series of experimental data sets of reflectivity against azimuthal angle (D) at wavelengths of (a) 7.5min, (b) 8.5 mm, (c) 9.5 min and (d) 10.5 min, showing the Rppy Rs, Rps and R,s signals respectively. The solid curves are the theoretical fits, which are in good agreement with the experimental data. During the fitting process, the amplitude of the corrugation, thickness and real part of the permittivity of the wax, and the polar angle of incidence are all allowed to vary from their measured values. The imaginary part of the permittivity of the wax is initially assumed to be zero, the pitch of the grating is;, = 15 nim and the permittiVities of the metal and air are assumed to be Erlletal = _106 + 106 i and Ps,,j, = 1.0 + O.Oi respectively. Distortion of the grating profile (a2, a3) is also introduced, however it does not improve the average quality of the fits.

A surface according to the invention provides a radar absorbing material for stealth applications, and with commercial applications in areas such as automotive and airport radar control. In prior art absorbers described a sufficiently large grating depth is required to shorten the lifetime of the mode and sufficiently widen the resonance so that it may be easily observed. Using a corrugated dielectric overlayer with non-zero Ei deposited on a planar metal surface, a second damping mechanism by which the SPP may decay is introduced and the need for such large corrugation amplitudes is decreased.

Figures 5 and 6 illustrate the effect of the imaginary part of the permittivity of the dielectric layer on the modelled R,s response and degree of absorption of the sample at 1 Inun wavelength. This shows the position of the modes in momentum-space does not change, but the width of these resonances is increased. In addition an absorbing overlayer will decrease the coupling strength to the SPP since the magnitude of the evanescent fields at the metal surface will be reduced. The introduction of absorption in the dielectric decreases the background reflectivity level, however the degree of absorption on-resonance of a wellcoupled mode is greatly enhanced. Figure 11 also illustrates the degree of absorption on a planar sample of the same mean thickness.

It would be understood that the dielectric profiled surface may be provided in alternative ways. The profile is preferably waveformed which includes sinusoidal, saw-tooth, triangular or rectangular wave forms. The amplitude and pitch of the grating would be geared according to the wavelengths to be absorbed, but would probably be between 0.5 and 2.0 times the 6 appropriate wavelength. As far as the thickness of the profile, it is preferrably less than a quarter of a wavelength.

The profiled dielectric layer may comprise parallel strips of suitable thin tape material. This embodiment has the advantage that the dielectric layer can be simply applied to existing surfaces.

Other variations include the dielectric layer having a checker board pattern. The advantage of this arrangement is that it provides for a regular pattern in two perpendicular axes on the plane in the surface.

The grating may alternatively comprise a hexagonal mesh of 'dots' or any other geometry. The advantage in higher symmetry groups is that they give a reduction in azimuthal and polarisation sensitivity. The repeat period could be single, multiple or variable to ensure broadband operation, and the entire surface could be 'capped' with a dielectric of a different permittivity to form a protective top-coat that presents a planar uppermost surface.

7

Claims

Claims

I A radiation absorber comprising a substrate having free charges and a dielectric layer coated onto said surface wherein the dielectric layer has a textured/pattemed surface.
2. A radiation absorber as claimed in claim 1 wherein said textured surface is located on the upper surface.
3. A radiation absorber as claimed in claims I or 2 wherein the textured surface is waveform.
4. A radiation absorber as claimed in any preceding claim wherein the dielectric layer comprises a plurality of tape strips.
5. A radiation absorber as claimed in any preceding claim wherein said dielectric layer has symmetry in at least two axes over the surface.
6, A radiation absorber as claimed in any preceding claim wherein said dielectric material includes doping agents.
7. A radiation absorber as claimed in any preceding claim further comprising a further coating over the dielectric material of different dielectric constant.
8. A building comprising a radiation absorber as claimed in any above claim.
9. A vehicle or aircraft comprising a radiation absorber as claimed in any of claims I to 7.
10. A solar panel comprising a radiation absorber as claimed in any of claims I to 7.
11, A method of reducing the radiation reflected/ retransmitted from an object by using a radiation absorber as claimed in any of claims I to 7.